Method for forming polarization reversal

ABSTRACT

A method for forming a ferroelectric spontaneous polarization reversal in a desired region of a ferroelectric substrate includes the steps of forming, for the desired region of the surface of the ferroelectric substrate, an electrode pattern or a mask pattern composed of aggregates of micropatterns, and then applying a given voltage into the desired region. The configuration of the micropatterns can be a stripe-shaped pattern, an ellipse-shaped pattern, a hexagon-shaped pattern, a network pattern, or a double cross shaped pattern. The method can further include the steps of generating many nucleuses by using the electrode pattern or the mask pattern composed of the aggregates of micropatterns, forming another electrode pattern or another mask pattern corresponding to the desired region, and then applying a given voltage into the desired region to generate a ferroelectric spontaneous polarization reversal around the nucleuses.

This application is a divisional of patent application Ser. No.11/083,735 filed Mar. 18, 2005, claiming priority of Japan Pat. App. No.2004-079224 filed Mar. 18, 2004; Japan Pat. App. No. 2004-089890 filedMar. 25, 2004; and Japan Pat. App. No. 2004-104323 filed Mar. 31, 2004,and contents of all of these applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a method for forming a ferroelectricspontaneous polarization reversal in a desired region of a ferroelectricsubstrate, and more particularly to a method for forming a ferroelectricspontaneous polarization reversal that forms on a substrate having anelectrooptic effect that is used for an optical element. In addition, itrelates to a method for forming a ferroelectric spontaneous polarizationreversal, where a ferroelectric substrate has convexo-concave structureon its surface and at a region including one portion of said convexpart. Further, it relates to a method for forming a ferroelectricspontaneous polarization reversal capable of reversing the polarity of anarrow region of a substrate and also, capable of narrowing the intervalbetween ferroelectric spontaneous polarization reversal regions.

(2) Related Art Statement

An optical element such as a wavelength conversion element or an opticalmodulator is used in optical communication and optical measurementsystems.

For example, a wavelength conversion element has periodicalferroelectric spontaneous polarization reversal structures on asubstrate with an electrooptic effect such as a ferroelectric LiNbO₃, asdisclosed in the following patent document 1.

Also, as an example of an optical modulator, the optical modulator whichhas optical waveguides on a substrate with an electrooptic effect andhas a ferroelectric spontaneous polarization reversal structure in oneportion of the substrate related to said optical waveguides to suppresschirp generation or to improve extinction ratio of modulation intensity,as disclosed in the following patent document 2, has been proposed.

-   [Patent Document 1] Japanese Patent Application Publication No.    2000-147584-   [Patent Document 2] Japanese Patent Application Publication No.    2003-202530

As a method for forming a ferroelectric spontaneous polarizationreversal region on a ferroelectric substrate, Ti thermal diffusiontechnique, heat treatment after loading SiO₂ technique, and a protonexchange and following heat treatment technique can be cited. Inaddition, a method for forming a ferroelectric spontaneous polarizationreversal region by applying an electric field higher than the value ofcoercive filed (e.g. 20 kV/mm for LiNbO₃) is also known.

Specifically, ferroelectric spontaneous polarization reversal byapplying an electric field is widely used as a method for forming aferroelectric spontaneous polarization reversal because it is possibleto form a ferroelectric spontaneous polarization reversal regionaccurately and the method for forming is simple.

As a method for forming a ferroelectric spontaneous polarizationreversal by electric field, it has been proposed to apply a voltage 4through electrodes 2 and 3 which are fabricated on the top and bottomfaces of substrate 1 as shown FIG. 1, or to apply voltage 4 throughelectrodes 6 and 7 where conductive liquid is filled between substrate 1and each electrode after fabricating insulating patterned mask 5 on thetop face of substrate 1 using sealing members 8 and 9 at the same timeas fixing said substrate as shown in FIG. 2. In addition, in case ofusing an insulating material such as an acrylic board instead ofelectrodes 6 and 7, it is arranged that an electric wire directlycontact the conductive liquid for feeding of voltage 4.

By these methods, the ferroelectric spontaneous polarization reversalregions corresponding to the pattern of electrodes 2 and ferroelectricspontaneous polarization reversal regions corresponding to the regionwhere the mask patterns 5 are not formed are formed respectively in FIG.1 and FIG. 2.

For the methods for forming a ferroelectric spontaneous polarizationreversal by electric field as stated above, when the width of aferroelectric spontaneous polarization reversal region, for example awidth L of a ferroelectric spontaneous polarization reversal region 10formed on substrate 1 in FIG. 3 is less than 20 μm, it is possible toform relatively homogeneous ferroelectric spontaneous polarizationreversal regions because a homogeneous voltage is applied all over theregion where a ferroelectric spontaneous polarization reversal is to beformed.

On the other hand, when a large ferroelectric spontaneous polarizationreversal region having width L of more than 50 μm is formed, it resultsin inhomogeneous ferroelectric spontaneous polarization reversalcondition as a whole because a ferroelectric spontaneous polarizationreversal is formed preferentially in the periphery of the region where aferroelectric spontaneous polarization reversal is expected to beformed. Also, it becomes difficult to form a ferroelectric spontaneouspolarization reversal homogeneously because of in-plane variation ofwafer thickness and differences in voltage effects that are caused bydispersion of electric resistance of electrodes that apply an electricfield. Thus, when a large diameter wafer is used, the differences offerroelectric spontaneous polarization reversal condition depending onin-plane location of the wafer become prominent.

In the meantime, the following non patent documents 1 to 6 disclose thata process for forming a ferroelectric spontaneous polarization reversalcomprises of the nucleation at the concentrated region of an electricfield of an electrode edge, the expanding of microdomain in a depthdirection so as not to increase electrostatic energy, the movement ofdomain wall in a transverse direction, and the stabilization of aferroelectric spontaneous polarization reversal region, and thesedocuments also disclose that the degree of nucleation density isimportant for homogeneity of a ferroelectric spontaneous polarizationreversal.

In other words, it can be easily understood that, when a large regionhaving width L of more than 50 μm for forming a ferroelectricspontaneous polarization reversal, the polarity of the periphery ispreferentially reversed, and that it is difficult to form a homogeneousferroelectric spontaneous polarization reversal when width L is widebecause the nucleation density is lower compared with when said width isnarrow.

Thus, the non patent documents 1 to 6 propose a method for applying lowelectric field pulses into a substrate to generate nucleuses that are tobe the nucleus of a ferroelectric spontaneous polarization reversal, andsubsequently applying a high electric field pulse to extend domain wallfrom said nucleus to thereby realize a ferroelectric spontaneouspolarization reversal.

Moreover, it has been reported that a homogeneous ferroelectricspontaneous polarization reversal region can be obtained by this method.

[Non Patent Document 1]

-   Sunao KURIMURA et al., “Selective nucleation control for a    periodically poled lithium niobate I ˜motivation and background˜”,    Pre-Texts of the 49th Meeting; The Japan Society of Applied Physics    and Related Societies, March 2002

[Non Patent Document 2]

-   Takeshi AKUTSU et al., “Selective nucleation control for a    periodically poled lithium niobate II-   ˜Periodical poling by selective nucleation control˜”, Pre-Texts of    the 49th Meeting; The Japan Society of Applied Physics and Related    Societies, March 2002

[Non Patent Document 3]

-   Masayuki MARUYAMA et al., “Selective nucleation control for a    periodically poled lithium niobate III ˜Qualification of nucleation    density by particle analysis˜”, Pre-Texts of the 49th Meeting; The    Japan Society of Applied Physics and Related Societies, March 2002

[Non Patent Document 4]

-   Yoshiyuki NOMURA et al., “Selective nucleation control for a    periodically poled lithium niobate IV ˜Pulse number dependence of    nucleation density˜”, Pre-Texts of the 63rd Meeting; The Japan    Society of Applied Physics, September 2002

[Non Patent Document 5]

-   Masayuki MARUYAMA et al., “Selective nucleation control for a    periodically poled lithium niobate V ˜Nucleation parameters for    short period structure˜”, Pre-Texts of the 63rd Meeting; The Japan    Society of Applied Physics, September 2002

[Non Patent Document 6]

-   Masayuki MARUYAMA et al., “Selective nucleation control for a    periodically poled lithium niobate VI ˜green light SHG with    high-aspect-ratio periodicdomains˜”, Pre-Texts of the 50th Meeting;    The Japan Society of Applied Physics and Related Societies, March    2003

Also, for an optical element such as an optical modulator, opticalelement having ridge structure has been put to practical use for thepurposes of lowering drive voltage, matching impedance, and expandingbandwidth.

FIG. 7( a) is a perspective view of the optical element having ridgestructure and FIG. 7( b) is a cross-sectional view of the substratealong the chain line A in FIG. 7( a). Modulating electrodes or the likeare not shown in these figures.

An optical waveguide 112 is formed on a ferroelectric substrate 101while a ridge 110 is formed on the region including said opticalwaveguide 112 in FIG. 7. Further, they show a ferroelectric spontaneouspolarization reversal region 111 formed on one portion of an opticalwaveguide 112.

A method for forming a ferroelectric spontaneous polarization reversalin one portion of the ferroelectric substrate and subsequently removinga region where a ridge structure is not formed on the top face of saidsubstrate selectively by mechanical cut or chemical etching as shown inFIG. 8 is a common method for forming a ferroelectric spontaneouspolarization reversal in a region having convexo-concave structure suchas a ridge structure on the top face of the ferroelectric substrate andincluding one portion of said convex part as shown in FIG. 7.

Regarding the specific procedures, at first, an electrode 120 having adesired pattern is formed on the top face of a ferroelectric substrate101 and an electrode 121 is formed on all over the bottom face of saidsubstrate as shown in FIG. 8( a). Then, a high voltage is appliedbetween said electrodes 120 and 121 by a voltage source 122 to form aferroelectric spontaneous polarization reversal 111 in the regioncorresponding to the pattern of electrode 120.

After that, electrodes 120 and 121 on the substrate are removed while amask is formed corresponding to the pattern of a ridge structure formedon the top face of substrate 101. The substrate top face except themask-covered region is removed chemically by dry etching or wet etching,or mechanically by sandblast or cutting to thereby form ridge structures123 (see FIG. 8( b)). The concavity and convexity on the substratesurface where an optical waveguide is not formed as shown in thecross-sectional view of FIG. 7( b) are not shown in FIG. 8 to FIG. 14 tofacilitate understanding of the description of the present invention.

However, it is difficult to form a desired ridge structure in thechemical removal process because the etching velocity and/or etchingdirection priority between the ferroelectric spontaneous polarizationreversal region and the other region is different, and also, there is adisadvantage that in a mechanical removal process the substrate getseasy to break because the substrate receives much of a shock entirely.

Thus, it is required to form the ridge structure on a ferroelectricsubstrate firstly, and then, to form a ferroelectric spontaneouspolarization reversal in a desired region of said substrate.

FIG. 9 show the method for forming a ferroelectric spontaneouspolarization reversal after forming a ridge structure.

At first, optical waveguides 130 are formed on the top face of substrate101 (see FIG. 9( a)), then, mask members 131 are formed on the regionfor forming a ridge structure on the top face of the substrate 101.After that, the region where mask members 131 are not formed is removedby etching or the like to thereby form ridge structures 132.

In order to form a ferroelectric spontaneous polarization inferroelectric substrate 101 which has formed ridge structures 132, atfirst an electrode 133 is formed on the desired region of the top faceof the substrate 101, and at the same time, an electrode 134 is formedall over the bottom face of the substrate 101. Then, a high voltage 135is applied between both electrodes 133 and 134 to form a ferroelectricspontaneous polarization reversal 136 in the region of the substratecorresponding to the pattern of electrode 133 as shown in FIG. 9( d).

After forming a ferroelectric spontaneous polarization reversal,electrodes 133 and 134 are removed to obtain the substrate having theridge structures where the polarity of one portion is reversed as shownin FIG. 9( e).

However, the ferroelectric spontaneous polarization reversal method inFIG. 9 has some negative effects such as the substrate becomes easy tobreak because the electric filed becomes concentrated at the edges 137of electrode 133 formed on the ridge structure in FIG. 9( d), andfurther, when forming a ferroelectric spontaneous polarization reversalover the ridge structure and other regions, voltage adjustment in aferroelectric spontaneous polarization reversal becomes complicatedbecause the ridge and other regions are different in thickness andtherefore, are different in strength of the electric field.

On the other hand, the method for using the insulating mask as shown inFIGS. 10 and 11 is cited as a method for forming a ferroelectricspontaneous polarization reversal in a ferroelectric substrate 101 whichformed the ridge structure in FIG. 9( c).

In FIG. 10, an insulating mask 140 is formed on the top face ofsubstrate 101 where a ferroelectric spontaneous polarization reversal isnot formed. Said substrate is put between electrodes 142 and 143 withsealing members 141 while conductive liquids 145 and 146 are filledbetween a substrate 101 and each electrode 142 and 143, and a highvoltage 144 is applied through the electrodes 142 and 143 like FIG. 2.

This method enables the forming of ferroelectric spontaneouspolarization reversal 147 in the region of the substrate where theinsulating mask 140 is not formed. The insulating mask is removed awayafter the ferroelectric spontaneous polarization reversal.

Also, in FIG. 11, an insulating mask 150 is formed on the bottom face ofsubstrate 1 where a ferroelectric spontaneous polarization reversal isnot formed. Said substrate is put between electrodes 152 and 153 withsealing members 151 while conductive liquids 155 and 156 are filledbetween a substrate 101 and each electrode 152 and 153, and a highvoltage 154 is applied through the electrodes 152 and 153 like FIG. 2 orFIG. 10.

After that, a ferroelectric spontaneous polarization reversal 157 in theregion of the substrate where the insulating mask 150 is not formed isformed, and subsequently the insulating mask 150 is removed.

However, the method, for forming a ferroelectric spontaneouspolarization reversal using the insulating mask as shown in FIGS. 10 and11 has a problem that the material for the insulating mask is limitedbecause it is necessary to select the material having electricresistance higher than that of the ferroelectric substrate 101.

Furthermore, although when a resist mask that is widely used as theinsulating mask is used here, hard baking treatment is effective inincreasing the electric resistance value of a resist mask, it becomesdifficult to remove the resist mask after forming a ferroelectricspontaneous polarization reversal. Moreover, the control accuracy of aferroelectric spontaneous polarization reversal region declines becauseunintended micro ferroelectric spontaneous polarization reversal regions(micro-domain) are introduced into the substrate entirely due to theheat treatment, and operation troubles or a degradation ofcharacteristics of a device using a ferroelectric spontaneouspolarization reversal structure, such as a wavelength conversion elementor an optical modulator, could be caused.

In addition, when an electric resistance value of an insulating maskcannot be raised adequately, a ferroelectric spontaneous polarizationreversal region sometimes spreads to an unintended region of thesubstrate as shown by 148 of FIG. 10 or 158 of FIG. 11. It becomesdifficult to control the formation of the ferroelectric spontaneouspolarization reversal region accurately.

Further, the method for forming a ferroelectric spontaneous polarizationreversal by the electrode pattern as shown in FIG. 1 or the insulatingmask pattern as shown in FIG. 2 has problems that the fineness and thecloseness of a ferroelectric spontaneous polarization reversal regionare limited depending on the formation accuracy of a stroke width and/ora spacing of each pattern, and that the production process including toform a ferroelectric spontaneous polarization reversal becomescomplicated because extra processes for forming patterns and removingthe patterns have to be needed in the production process.

The first object of the present invention is to solve the problemsdescribed above and to provide a method for forming a ferroelectricspontaneous polarization reversal homogeneously even if the width of aferroelectric spontaneous polarization reversal region is over 50 μm,and further, to provide a method for forming a ferroelectric spontaneouspolarization reversal that enable the lowering of the intensity of anapplied voltage in a ferroelectric spontaneous polarization reversal.

The second object of the present invention is to provide a method forforming a ferroelectric spontaneous polarization reversal where theferroelectric substrate has convexo-concave structure, such as ridgestructure and the like, on its surface and the polarity of the regionincluding one portion of said convex part is reversed with accuracy.

Moreover, the third object of the present invention is to provide amethod for forming a ferroelectric spontaneous polarization reversalwhich could form a polarization reversal region closely, accurately andfinely on a ferroelectric substrate, and also to provide the method forforming a ferroelectric spontaneous polarization reversal that iscapable of preventing a production process from being complicated.

SUMMARY OF THE INVENTION

In order to solve the problems described above, a first aspect of theinvention provides the method for forming a ferroelectric spontaneouspolarization reversal in a desired region of a ferroelectric substrate,which has a feature that said desired region of a surface of saidferroelectric substrate is sprayed with micro-hard materials, made animpact by using a striking member that has micro tip diameter, or rubbedwith micro-hard materials that are dispersively located on the surfaceof the substrate, and subsequently, a given voltage is applied into saiddesired region to thereby form a ferroelectric spontaneous polarizationreversal in said desired region of said ferroelectric substrate.

A second aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal in a desired region of aferroelectric substrate, wherein, for said desired region of a surfaceof the ferroelectric substrate, an electrode pattern or a mask patternis composed of aggregates of micropatterns, and subsequently a givenvoltage is applied into said desired region.

A third aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal in a desired region of aferroelectric substrate, where the thickness of substrate in saiddesired region of said ferroelectric substrate is thinner than theregion except said desired region of the substrate, and a given voltageis applied into said desired region to thereby form a ferroelectricspontaneous polarization reversal in said desired region of saidferroelectric substrate.

A fourth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to the thirdaspect, which has a feature that a surface of the ferroelectricsubstrate is etched in order to make said ferroelectric substratethinly.

A fifth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to any of thethird and fourth aspects, which has a feature that the face for formingsaid ferroelectric substrate thinly is different from the face forforming a waveguide on said ferroelectric substrate.

A sixth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal, where a convexo-concavestructure is formed on the top face of a ferroelectric substratefirstly, and then, a ferroelectric spontaneous polarization region isformed on the substrate including one portion of said convex part, whichhas a feature that a concave portion is formed on the bottom face of thesubstrate within the region where a ferroelectric spontaneouspolarization reversal is to be formed and at least said convex portionis formed, and then, an electric field is applied into said substrate.

A seventh aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to the sixthaspect, having a feature that the depth of said concave portion on thebottom face of the substrate is greater than the height of the convexportion on the top face of the substrate.

An eighth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to any of thesixth and seventh aspects, which has a feature that the width of saidconcave portion on the bottom face of the substrate is wider than thewidth of said convex portion on the top face of the substrate.

A ninth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to any of thesixth through eighth aspects, which has a feature that said electricfield is applied via liquid electrode.

A tenth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to any of thesixth through ninth aspects, which has a feature that at least oneportion of the region except said concave portion on the bottom face ofthe substrate is removed after forming a ferroelectric spontaneouspolarization reversal.

An eleventh aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to any of thesixth through ninth aspects, which has a feature that said concaveportion on the bottom face of the substrate is filled entirely orpartially with a material that has almost equal dielectric constant orcoefficient of thermal expansion of said substrate after forming aferroelectric spontaneous polarization reversal.

A twelfth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal where a concave portionis formed on the top face of a ferroelectric substrate or the bottomface of a ferroelectric substrate, an electric field is applied into thesubstrate, and a ferroelectric spontaneous polarization reversal isformed at least in one portion of a region of said substrate with saidconcave portion, which has a feature that the shape of said concaveportion is configured such that the width of said concave portion getsnarrower gradually toward the inside of the substrate.

A thirteenth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to the twelfthaspect, which has a feature that after first forming of a ferroelectricspontaneous polarization reversal, the top face of the substrate or thebottom face of the substrate that has already formed said concaveportion is made almost a flat-plane, and then, a new concave portion isformed in a region where it was not formed previously, and after that anelectric field is applied into said substrate to form a ferroelectricspontaneous polarization reversal again at least in one portion of theregion of the substrate with said concave portion.

A fourteenth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to the twelfthaspect, which has a feature that the concave portions formed on the topand bottom faces of said substrate are placed in different geometriclocations where the following ferroelectric spontaneous polarizationreversal regions corresponding to said concave portions could notoverlap.

A fifteenth aspect of the invention provides the method for aferroelectric spontaneous forming polarization reversal according to anyof the twelfth through fourteenth aspects, which has a feature that saidapplied electric field is gradually increased from a weak electric fieldincluding 0 to the given value of the electric field.

A sixteenth aspect of the invention provides the method for forming aferroelectric spontaneous polarization reversal according to the twelfththrough fifteenth aspects, which has a feature that said electric fieldis applied via liquid electrode.

In accordance with the first aspect of the invention, nucleuses aregenerated on the substrate surface by adding mechanical stress to thesubstrate surface using the methods for spraying the desired region ofthe ferroelectric substrate surface with the micro-hard materials,making an impact by the striking member that has micro tip diameter, orrubbing with the micro-hard materials that are dispersively located onthe substrate surface, or the like.

Subsequently, centering on the nucleuses, a ferroelectric spontaneouspolarization reversal is formed by applying the given voltage to saiddesired region.

This makes it possible to generate nucleuses within the region where itis desired to form a ferroelectric spontaneous polarization reversal aswell as its periphery and to improve nuclear growth density. It ispossible to achieve a homogeneous ferroelectric spontaneous polarizationreversal even when a large dimension of a ferroelectric spontaneouspolarization reversal region is formed.

Furthermore, for a LiNbO₃ crystal or a LiTaO₃ crystal, nucleuses on thesubstrate surface can be generated by a minute stress or impact, andtherefore, a ferroelectric spontaneous polarization reversal region canbe formed easily without using a large-scale equipment. Moreover, sinceit is very little nucleuses that are formed on the substrate surface andthe mechanical load on the substrate is small as a whole, the mechanicalproperty of the substrate itself does not deteriorate as a result.

In accordance with the second aspect of the invention, the use ofmicropatterns can generate nucleuses around each pattern, and,considering the whole polarization reversal region, it is possible togenerate many nucleuses within the region. Even when a large dimensionof a ferroelectric spontaneous polarization reversal region is formed,homogeneous polarization reversal can be realized.

In accordance with the third aspect of the invention, to make thethickness of the region of the substrate that is forming a ferroelectricspontaneous polarization reversal thinly enables to reduce appliedvoltage for forming a ferroelectric spontaneous polarization reversal.This method is effective to avoid troubles such as dielectric breakdownand this could also reduce the cost for mass production by degreasingrequirements for the special fixture and/or power supply equipment.

In accordance with the fourth aspect of the invention, in the thinningprocess of said ferroelectric substrate thickness, there is nomechanical load on the substrate as a whole because etching is appliedto the surface of said ferroelectric substrate and therefore themechanical property of the substrate itself does not deteriorate as aresult. Also, when a waveguide element is formed, it is desirable toremove the step that is formed after a ferroelectric spontaneouspolarization reversal by polishing or the like because said step couldcause an optical loss.

In accordance with the fifth aspect of the invention, it could providean optical waveguide device that has no degradation of opticalcharacteristics such as an optical loss because there is no affect ofchanging the substrate thickness because although one region of one faceof the substrate corresponding to the shape of the ferroelectricspontaneous polarization reversal region is thinned by cut or etching,the optical waveguide is formed on the other face of the substrate. Inthis case, the process stated above of removing the step by polishingcan be omitted.

In accordance with the sixth aspect of the invention, it becomespossible to increase the strength of an electric field in forming aferroelectric spontaneous polarization reversal because a concaveportion is formed on the bottom face of the ferroelectric substrate in aregion where the convex portion is formed and the substrate in theregion with the convex portion is thinned.

Thus, it becomes possible to form a polarization reversal accurately insaid region of the substrate including said convex part.

In accordance with the seventh aspect of the invention, by making thedepth of the concave portion formed on the bottom face of the substrategreater than the height of the convex portion formed on the top face ofthe substrate, the strength of an electric field applied to the regionof the convex part on the substrate top face in forming polarizationreversal can be equalized with or be made stronger than the strength ofother regions where said concave portion is not formed on the bottomface of the substrate. Thus, it becomes possible to form polarizationreversal accurately in said region of the substrate including saidconvex part.

In accordance with the eighth aspect of the invention, by making thewidth of the concave portion formed on the bottom face of the substratewider than the width of the convex portion formed on the top face of thesubstrate, it becomes possible to form a ferroelectric spontaneouspolarization reversal where said region of the substrate including theconvex portion certainly. Moreover, it is possible to control aferroelectric spontaneous polarization reversal region more clearlybecause the bottom edges of the convex portion and the bottom of theconcave portion are located proximally, and as a result, a ferroelectricspontaneous polarization reversal is formed in the early stage offorming a ferroelectric spontaneous polarization reversal in thisproximal section, and after that, a ferroelectric spontaneouspolarization reversal makes progress in the region of the convex part.

In accordance with the ninth aspect of the invention, it is possible toapply an electric field all over the ferroelectric substratecorresponding to the thickness of the substrate, irrespective ofconvexo-concave structure on the top face or the shape of the convexportion on the bottom face of the substrate because a ferroelectricspontaneous polarization reversal is performed by the liquid electrodeusing conductive liquid. Thus, the ferroelectric spontaneouspolarization reversal region can be controlled accurately by setting theshape of the top and bottom faces of the substrate with accuracy.

In accordance with the tenth aspect of the invention, by removing atleast one portion of the region except said concave portion on thebottom face of the substrate after forming a ferroelectric spontaneouspolarization reversal, it is possible to compensate a stressdistribution over the ferroelectric substrate caused by a temperaturechange or a propagation characteristics change of a microwave or thelike in case of using the ferroelectric substrate as an optical device.

In accordance with the eleventh aspect of the invention, by filling thewhole or a portion of the concave portion on the bottom face of thesubstrate with the material having almost equal dielectric constant orcoefficient of thermal expansion of said substrate after forming aferroelectric spontaneous polarization reversal, it is possible tocompensate the electrical characteristic of the substrate or the stressdistribution related to the thermal expansion by adjusting thedielectric constant or a spatial distribution of coefficient of thermalexpansion of the substrate.

In accordance with the twelfth aspect of the invention, since the shapeof the concave portion is configured such that the width of said concaveportion gets narrower gradually toward the inside of the substrate, thethickness of the substrate varies according to location within theconcave portion. It is configured such that the thickness of thesubstrate gets thicker gradually from the deepest part of the concaveportion toward the entry of the concave portion. Due to thisconfiguration, it becomes possible to arbitrarily form a ferroelectricspontaneous polarization reversal region in accordance with the strengthof an electric field applied into the substrate, centering on thedeepest part of the concave portion and also within the width of theentry of the concave portion.

In other words, it becomes possible to form a ferroelectric spontaneouspolarization reversal region with its width narrower than the entryshape of the concave portion.

In accordance with the thirteenth aspect of the invention, it couldbecome possible to locate and form several ferroelectric spontaneouspolarization reversal regions closely by using the method that firstlyforming a ferroelectric spontaneous polarization reversal using theconcave portion of the substrate, then, new concave portion is formedagain on the region except said concave portion had formed previously.

In accordance with the fourteenth aspect of the invention, it couldbecome possible to locate and form several ferroelectric spontaneouspolarization reversal regions closely because the polarization reversalregion formed by the concave portion on the top face of the substrateand the polarization reversal region formed by the concave portion onthe bottom face of the substrate are located at different places withinthe substrate.

In accordance with the fifteenth aspect of the invention, it is possibleto arbitrarily adjust a ferroelectric spontaneous polarization reversalregion in accordance with the given strength of an electric fieldapplied into the substrate, centering on the deepest part of the concaveportion and also within the width of the entry of the concave portion,in such a manner that the electric field applied into the substrate iscontrolled to be gradually increased from the weak electric fieldincluding 0 to a given strength of an electric field in forming aferroelectric spontaneous polarization reversal.

In accordance with the sixteenth aspect of the invention, it is possibleto apply an electric field into the substrate effectively even when theconcave portion is formed on the top face or bottom face of thesubstrate because the electric field is applied via the liquidelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the method for forming a ferroelectricspontaneous polarization reversal using the electrode pattern accordingto the prior art;

FIG. 2 is a diagram showing the liquid electrode method according to theprior art;

FIG. 3 is a diagram showing the ferroelectric spontaneous polarizationreversal region formed on the substrate;

FIG. 4 is a diagram showing the micro-defects or the like formed on thesurface of the substrate;

FIG. 5 are diagrams where a mask pattern is composed of aggregates ofmicropatterns;

FIG. 6 is a diagram showing a ferroelectric spontaneous polarizationreversal being performed for the thinned substrate by the liquidelectrode method;

FIG. 7 are diagrams showing the ferroelectric substrate having ridgestructures; (a) is a perspective view of said substrate and (b) is across-sectional view of said substrate;

FIG. 8 are diagrams showing the example in which ridge structures areformed after a ferroelectric spontaneous polarization reversal accordingto the prior art;

FIG. 9 are diagrams showing examples in which polarization reversal isperformed after forming ridge structures according to the prior art;

FIG. 10 is a diagram showing the example in which the insulating mask isformed on the surface of the ferroelectric substrate according to theprior art and a ferroelectric spontaneous polarization reversal isperformed;

FIG. 11 is a diagram showing the example in which the insulating mask isformed on the bottom face of the ferroelectric substrate according tothe prior art and a ferroelectric spontaneous polarization reversal isperformed;

FIG. 12 is a diagram showing the concave portion being formed on thebottom face of the ferroelectric substrate of the present invention;

FIG. 13 is a diagram showing the method for forming a ferroelectricspontaneous polarization reversal for the ferroelectric substrate havingthe concave portion on the bottom face of the substrate according to thepresent invention;

FIG. 14 are diagrams showing the treatment after a ferroelectricspontaneous polarization reversal for the ferroelectric substrate havingthe concave portion on the bottom face of the substrate; (a) is adiagram in which the region except the concave portion is removed and(b) is a diagram in which the whole or a portion of said concave portionis filled in;

FIG. 15 are diagrams showing the example of the method for forming aferroelectric spontaneous polarization reversal according to the presentinvention; (a) is a cross-sectional view of the structure of thesubstrate before a ferroelectric spontaneous polarization reversal and(b) is a diagram showing the condition after a ferroelectric spontaneouspolarization reversal;

FIG. 16 are diagrams showing the method for forming a fine and closeferroelectric spontaneous polarization reversal regions by the methodfor forming a ferroelectric spontaneous polarization reversal accordingto the present invention; and

FIG. 17 are diagrams showing another method for forming a fine and closeferroelectric spontaneous polarization reversal region by the method forforming polarization reversal according to the present invention.

EXPLANATIONS OF NUMERALS

-   -   1, 101, 201 Ferroelectric substrate    -   2 Patterned electrode    -   3 Bottom face electrode    -   4 Applied voltage    -   5 Insulating mask pattern    -   6, 7, 162, 163 Electrode    -   8, 9, 161 Sealing member    -   10, 167, 211, 221, 224, 232, 233 Ferroelectric spontaneous        polarization reversal region    -   20 Micro-defects or micro-residual stress region    -   130 Optical waveguide    -   132 Ridge structure    -   160, 210, 220, 223, 230, 231 Concave portion    -   165, 166 Conductive liquid    -   167 Ferroelectric spontaneous polarization reversal region    -   170 Removed portion    -   71 Filled part

DETAILED DESCRIPTION OF THE INVENTION

In the following, the preferred embodiments of the present invention areexplained in detail.

FIG. 4 is a diagram showing many nucleuses generated on the surface of aferroelectric substrate 1 for a ferroelectric spontaneous polarizationreversal region 20.

If such nucleuses exist, a ferroelectric spontaneous polarizationreversal is expanded around these nucleuses when an electric field isapplied into the ferroelectric substrate. Thus, if these nucleuses existmoderately in a desired ferroelectric spontaneous polarization reversalregion, it becomes possible to realize homogeneous ferroelectricspontaneous polarization reversal state across a large diameterferroelectric wafer, even when a ferroelectric spontaneous polarizationreversal is simultaneously performed for several large regions of over25 μm in a ferroelectric spontaneous polarization reversal region widthwithin a large diameter ferroelectric wafer of over 2 inches indiameter.

A method of applying mechanical shock or the like or a chemical methodcan be used as the method for generating nucleuses. In particular, whenmicro-defects or the like are formed by using mechanical power,nucleuses can be generated more effectively by raising the temperatureof the substrate.

In the following, specific methods are described.

(1) Micro Sandblast Method

A non-polarization reversal region on the surface of a substrate iscoated with a rubber resist film or the like, and then, sprayingabrasive material, dry ice or the like, that are micro-hard materials,with said surface of the substrate at high speed, which generatesnucleuses by these impact.

(2) Wire Brush Method

Like the micro sandblast method, the surface of a substrate is coatedwith a rubber resist film or the like, and then, a striking memberhaving micro tip diameter such as a wire brush gives said substratesurface a shock and thereby generates nucleuses.

(3) Application of a Dot Impact Printer

A printer head of a wire dot impact printer selectively gives a shock tothe surface of a substrate for a ferroelectric spontaneous polarizationreversal region, and thereby generates nucleuses.

(4) Application of a Printing Method

By a screen printing, an inkjet printer or the like, the pattern of apolarization reversal region is printed on the surface of a substrate byusing resin ink that comprises hard resin and abrasive material. Afterdrying the print, the printed part is rubbed with a baren or the likethrough a cushioning such as a cloth or paper to generate nucleuses.

(5) Application of an Abrasive Sheet or the Like

A non-polarization reversal region on the surface of a substrate iscoated with a rubber resist film or the like, and then, an abrasivesheet is pressed on or rubbed against the surface of the substrate.Alternatively, after said coating, abrasive material is sprayed over thesurface of the substrate and then rubbed against the surface of thesubstrate as being pressed on it. Further, a patterned abrasive sheet inthe shape of a ferroelectric spontaneous polarization reversal region ispreliminarily prepared. Said abrasive sheet is attached to the substrateand pressed on the substrate.

After generating nucleuses as stated above, it is possible to form aferroelectric spontaneous polarization reversal region on the substrateby applying the method for forming a ferroelectric spontaneouspolarization reversal as FIG. 1 or FIG. 2.

In particular, an electrode pattern corresponding to the shape of theferroelectric spontaneous polarization reversal region is formed on thetop face of the substrate and an electrode is uniformly formed on thebottom face of the substrate. By applying a pulse voltage between bothelectrodes, a ferroelectric spontaneous polarization reversal isexpanded around nucleuses. Thus, a homogeneous ferroelectric spontaneouspolarization reversal corresponding to said electrode pattern can beformed.

In a method using conductive liquid, after forming an insulating maskpattern on the surface of the substrate, conductive liquid is placed onboth faces of said substrate and a pulse voltage is applied into saidconductive liquid. This enables a ferroelectric spontaneous polarizationreversal to be expanded around nucleuses.

In addition, it is also possible to substitute the above rubber resistfilm used in generating nucleuses for the above insulating mask pattern.

As another method for forming a large ferroelectric spontaneouspolarization reversal region, there is a method of using the electrodepattern of in FIG. 1 or the mask pattern in FIG. 2 of aggregates ofmicropatterns. In the following, the mask pattern is explained as anexample.

Specifically, when a ferroelectric spontaneous polarization reversalregion of more than 20 μm in width as shown in FIG. 5( a) is formed, aferroelectric spontaneous polarization reversal region in a given shapeis formed as an aggregate of micro stripe-shaped mask patterns of 4 μmin width (an aperture being formed at a stripe-shaped portion of aninsulating mask) as FIG. 5 (b) firstly. Then, by contacting conductiveliquid with the ferroelectric substrate through said stripe-shapedaperture as shown in FIG. 2, an electric field is applied into saidsubstrate.

By using these micropatterns, nucleuses could develop on around eachpattern, and as a whole a ferroelectric spontaneous polarizationreversal region, it becomes possible to generate many nucleuses withinthe region. Beyond the stroke width in embodiments, nucleuses can begenerated stably if the stroke width of each pattern is less than 10 μm.

Also, as the configuration of the micropatterns, various applicationssuch as the ellipse-shaped pattern as in FIG. 5( c), hexagon-shapedpattern as in FIG. 5( d), network pattern as in FIG. 5( e), and further,double cross shaped pattern (or “lozenge-shaped”) as in FIG. 5( f) arepossible of applying as well as the stripe-shaped pattern as in FIG. 5(b). In addition, the numerals attached to each pattern here indicatewidth in mm unit.

Further, when straight line is comprised in the micropatterns, it isdifficult to cause merging of the ferroelectric spontaneous polarizationreversal region in some case of the crystal orientation of theferroelectric substrate. Thus, it is necessary to take intoconsideration the angle between straight line and the crystalorientation.

It is also possible to generate a ferroelectric spontaneous polarizationreversal around nucleuses by not only using the micropatterns as statedabove, but also by firstly performing nucleuses with the micropatternsand subsequently applying an electric field through a normal electrodepattern or mask pattern as well as reversing polarity of the wholeregion for forming a ferroelectric spontaneous polarization.

In addition, another method for forming a ferroelectric spontaneouspolarization reversal is explained.

In particular, to make the substrate thickness thinly in a region of aferroelectric spontaneous polarization reversal being desired enables tomake the electric field of the desired region higher than that of anon-desired region. As a result, without using a mask or the like, aferroelectric spontaneous polarization reversal can be performed only insaid desired region.

Especially by treating the substrate of a ferroelectric spontaneouspolarization reversal region with thinning processing, reduction of theapplied voltage can be achieved. It is preferred to thin the substratefrom its bottom face corresponding to the pattern of the ferroelectricspontaneous polarization reversal region and apply a voltage usingconductive liquid.

As the method for thinning, a non-a ferroelectric spontaneouspolarization reversal region of the substrate bottom face is coveredwith a rubber resist film or the like, and then, micro-hard materials,its sand scale being around #600, is sprayed and the substrate is dugfrom submicron to about 25 μm in depth. Subsequently, applying a voltageinto the substrate by the liquid electrode method as in FIG. 6 forms aferroelectric spontaneous polarization reversal region.

In this case, although the same value of voltage is applied into thethinned part A, where a ferroelectric spontaneous polarization reversalbeing desired, and the non-thinned part B, where a ferroelectricspontaneous polarization reversal not being desired, the effectiveelectric field “a” of A part is stronger than the electric field “b” ofB part. Thus, when said voltage and the thickness of thinned part A areadjusted such that A part has the electric field enough to enhance aferroelectric spontaneous polarization reversal and B part has theelectric field not enough to enhance a ferroelectric spontaneouspolarization reversal, it is possible to form a ferroelectricspontaneous polarization reversal in only said desired part.

Also in this case, a ferroelectric spontaneous polarization reversalcannot be generated in B part even if an electric field is applied for along time. Therefore, the desired ferroelectric spontaneous polarizationreversal region can be obtained without using accurate time control ofthe applied electric field by monitoring a poling current or the like.Further, the electric filed value necessary for forming a ferroelectricspontaneous polarization reversal become lower than the value whenthinning is not performed.

Hydrofluoric acid chemical etching or dry etching such as ECR, ISM, andNLD can be applied as another method for thinning the substrate.

At the hydrofluoric acid chemical etching, for example, it is performedafter coating, with polyimide resist et al., the bottom face of thesubstrate in the region where a ferroelectric spontaneous polarizationreversal is not desired. The depth of etching is from submicron to aboutseveral μm. Subsequently, a voltage is applied into the substrate by theliquid electrode method.

As a method for applying a voltage, a voltage is applied in the same wayof that the substrate is thinned by the micro hard materials describedabove. However, it is also possible to form a ferroelectric spontaneouspolarization reversal region by applying a higher voltage than thenormal ferroelectric spontaneous polarization reversal all over thesubstrate firstly before thinning the substrate, and then, reversing apolarity of the whole substrate once, and subsequently performing saidthinning treatment and applying the antipolarity voltage. It becomespossible to form a more homogeneous a ferroelectric spontaneouspolarization reversal region in this case.

The method for forming a ferroelectric spontaneous polarization reversalof the present invention can be also applied to forming an opticalwaveguide on the ferroelectric substrate and thereby preparing anwaveguide element such as an optical modulator or the like.

When using the above thinning of the substrate to form a ferroelectricspontaneous polarization reversal region, and further to form awaveguide on said thinned substrate, it is desirable to remove a step,which is made by the thinning process on the surface of the substrate,by polishing or the like because it cause an optical loss.

Also, when one face of the ferroelectric substrate corresponding to aferroelectric spontaneous polarization reversal region is thinned bycut, erosion and the like and the optical waveguide is formed on theother face of the substrate, it becomes possible to provide a waveguideelement without any degradation of characteristics such as an opticalloss because the variation of the substrate thickness never influencesthe optical waveguide at all. In this case, it is possible to produce awaveguide element even if a formation of a ferroelectric spontaneouspolarization reversal region and a formation of the optical waveguidebecome reverse in order.

The present invention is not limited to the above description, but it ispossible to combine various methods, such as using both nucleuses andthinning of a substrate for the micro sandblast method.

It is also possible to combine a method of irradiating a substrate withultraviolet ray at the time of applying a voltage and a method of dopinga impurity preliminarily on a region corresponding to the aferroelectric spontaneous polarization reversal region of a substrate asthe method for lowering the applied voltage for forming a ferroelectricspontaneous polarization reversal, according to need.

Subsequently, as the second object of the present invention, a methodfor forming a ferroelectric spontaneous polarization reversal wherein aferroelectric substrate has convexo-concave structure, such as ridgestructure and the like, on its surface and a polarity of a regionincluding one portion of said convex part is reversed with accuracy isexplained.

This invention has a feature that it could provide the method forforming a ferroelectric spontaneous polarization reversal capable ofreversing with accuracy in the region including one portion of a convexpart, when convexo-concave structure such as ridge structure is formedpreliminarily on the surface of the substrate.

Specifically, ridge structures 132 are formed on a top face of aferroelectric substrate 101 as shown in FIG. 12. When ridge structures132 are included in a region where a ferroelectric spontaneouspolarization reversal is to be formed, a concave portion 160 is formedon the bottom face of the substrate where said ridge structure is formedfirstly, and then, adjust the strength of an applied electric field informing a ferroelectric spontaneous polarization reversal. Here, 130indicates optical waveguides 130 formed on the ridge structures.

The shape of the concave portion is set to H≧h, preferably H>h bycomparing depth H of the concave portion with height h of the ridge andto W≧w by comparing width W of the concave portion with width w of theridge.

This makes it possible to configure the region of the convex part on asubstrate to have the same thickness with or to be thinner than theother region of a substrate even when the convex portion such as theridge structure is formed on the region of the substrate where aferroelectric spontaneous polarization reversal is to be formed. Thus,it becomes possible to control the ferroelectric spontaneouspolarization reversal region with accuracy because the strength of anelectric field at said region of the convex part is the same with orstronger than the other region when a voltage is applied in forming aferroelectric spontaneous polarization reversal.

It is more preferable to set width W of the concave portion 1.0 to 1.5times as wide as width w of the ridge structure.

In case of W<1.0 w, it becomes difficult to form a ferroelectricspontaneous polarization reversal region across the width of the ridgestructure.

On the other hand, in case of W>1.0 w, the substrate at the foot of theridge structure w is thinner than the region of the ridge structure andthe electric field of forming a ferroelectric spontaneous polarizationreversal gets stronger at these feet than at the region of the ridgestructure. As a result, a polarity of these feet regions ispreferentially reversed. Subsequently, a ferroelectric spontaneouspolarization reversal is formed also around the ridge region and itbecomes possible to control a ferroelectric spontaneous polarizationreversal region more clearly.

However, in case of W>1.5 w, regions thinner than the ridge region areformed around the ridge region. Then, a ferroelectric spontaneouspolarization reversal is expanded centering on these peripheries and aferroelectric spontaneous polarization reversal is formed outside of theridge region as well as in the ridge region. Thus, it is difficult tocontrol a ferroelectric spontaneous polarization reversal region onlywithin the ridge region accurately. However, this is not the case when aferroelectric spontaneous polarization reversal is needed to be formedmore widely including the peripheral region of the ridge structure dueto configuration of an optical modulator for example.

As a method for forming a concave portion on the bottom face of asubstrate, micro-hard materials, sand scale being around #600, aresprayed to dig it more than the height h of the ridge structure on thebottom face of the substrate coating the region where it is not desiredto form a concave portion with a rubber resist film or the like.

In addition, a hydrofluoric acid chemical etching or a dry etching suchas ECR and ICP can be applied. For example, at the hydrofluoric acidchemical etching, the region where it is not desired to form aferroelectric spontaneous polarization reversal region on the bottomface of a substrate is coated with polyimide resist and the hydrofluoricacid etching is performed.

In order to reverse a polarity of a ferroelectric substrate with aconcave portion formed on the bottom face of the substrate as shown inFIG. 12, the substrate in FIG. 12 is put between electrodes 162 and 163with sealing members 161 while conductive liquids 165 and 166 are filledbetween said substrate and each electrode 162 and 163, and then, a highvoltage 164 is applied into electrodes 162 and 163 as shown in FIG. 13.

As stated above, a conventional insulating mask is not necessary inreversing a polarity of a ferroelectric substrate and it is possible toskip a complicated process such as a formation and removing of theconventional insulating mask. Thus, the production process can besimplified. Further, a hard baking process for improving specificresistance in using resist as the insulating mask is not necessary, andtherefore, microdomain is not generated.

In addition, the present invention is not limited to the example thatdoes not use the insulating mask as shown in FIG. 13, but it is possibleto use the insulating mask as in FIGS. 10 and 11 to the ferroelectricsubstrate in FIG. 12 if necessary.

In this case, because a concave portion is formed on the bottom face ofthe substrate and the strength of an electric field on the ridge regionof the substrate can be increased, various materials can be applied asthe insulating mask while thermal process for improving the specificresistance of the insulating mask is not required. Thus, it is veryconvenient to use the present invention.

Next, handling of the ferroelectric substrate after forming aferroelectric spontaneous polarization reversal is explained.

It is possible to apply the ferroelectric substrate to various uses witha concave portion being formed on the bottom face of the substrate.However, in the case of changing the substrate temperature, for example,a deterioration of a mechanical property or an electrooptic property ofthe ferroelectric substrate could be caused because the distribution ofthermal stress that is caused by the partly formed concave portion onthe substrate gets inhomogeneous. Also, when an electric field isapplied into the ferroelectric substrate by a microwave that isfrequently used for an optical element such as an optical modulator, itis concerned that the propagation characteristic of the microwavechanges due to said concave portion.

In order to solve the problem due to the concave portion on the bottomface of the substrate, at least one portion of the bottom face of thesubstrate except the region where the concave portion is formed isremoved to thereby relax the influence of the concave portion on thebottom face of the substrate as shown in FIG. 14( a).

In addition, it is also possible to relax the influence of the concaveportion on the bottom face of the substrate by filling the concaveportion on the bottom face of the substrate entirely or partially with amaterial that has almost equal dielectric constant or coefficient ofthermal expansion of said substrate as shown in FIG. 14( b).

The present invention is not limited to the above description, butrather, it is obviously possible to apply a well known technology in theart if necessary.

Subsequently, as the third object of the present invention, a method forforming a ferroelectric spontaneous polarization reversal capable offorming a ferroelectric spontaneous polarization reversal region closelyand accurately on a ferroelectric substrate, and further, capable ofpreventing the production process from being complicated is explained.

The present invention is characterized in the method for forming aferroelectric spontaneous polarization reversal which forms a concaveportion on the top face or bottom face of the ferroelectric substrateand applies an electric field into said substrate to thereby reverse apolarity of at least one portion of the substrate where said concaveportion is formed, wherein the shape of said concave portion isconfigured such that the width of said concave portion gets narrowergradually toward the inside of the substrate.

Specifically, a concave portion 210 is formed on the bottom face of thea substrate 201 and said concave portion is configured such that thewidth of concave portion 210 gets narrower gradually toward the insideof the substrate as shown in FIG. 15( a). FIG. 15( a) is across-sectional view of ferroelectric substrate 201.

The ferroelectric substrate 201, which is processed as in FIG. 15( a),is put between electrodes 6 and 7 with sealing members 8 and 9 whileconductive liquid is filled between substrate 201 and each electrode 6and 7, and then, a voltage 4 is applied into electrodes 6 and 7 as shownin FIG. 2.

In this case, it is not necessary to form an insulating mask pattern onthe top or bottom face of the substrate 201 as shown in FIG. 2. However,the present invention does not eliminate the possibility of adding theinsulating mask pattern according to need.

In applying a voltage into substrate 201, the deepest part of a concaveportion 210 has the strongest electric field since it is the thinnest inthe substrate, and the electric field gets weaker and weaker toward theentry of the concave portion.

Thus, it becomes possible to arbitrarily form a ferroelectricspontaneous polarization reversal region centering on the deepest partof the concave portion and also within the width of the entry of theconcave portion by setting an appropriate applied voltage value.

Specifically, a ferroelectric spontaneous polarization reversal region211 is formed around the deepest part of the concave portion when anelectric field is weak (when an applied voltage is low), and aferroelectric spontaneous polarization reversal can be formed across aperipheral region 212 centering on said deepest part of the concaveportion when an electric field gets stronger (when an applied voltage ishigh) as shown in FIG. 15( b).

Therefore, by applying a weak electric field including 0 into thesubstrate and increasing the strength of the electric field graduallytill a given strength of an electric field in forming a ferroelectricspontaneous polarization reversal, it becomes possible to arbitrarilyadjust a ferroelectric spontaneous polarization reversal regioncentering on the deepest part of the concave portion and also within thewidth of the entry of the concave portion in accordance with the givenstrength of the electric field applied into the substrate.

Processing by sandblast or mechanical cut using a grinder can form aconcave portion on a substrate 201. It is also possible to form it bychemical treatment such as dry etching. At sandblast or dry etching, theregion where a concave portion is not formed is coated with a resistfilm and after that processed.

In applying an electric field into a substrate 201, it is necessary tofully exclude bubble at the concave portion of substrate 201preliminarily. It is preferable to perform an adequate degassing processby applying an ultrasonic wave into the substrate or conductive liquidif necessary.

Next, a method for forming a fine and close ferroelectric spontaneouspolarization reversal region is explained.

FIG. 16 are diagrams showing the process for forming a ferroelectricspontaneous polarization reversal region.

Concave portions 220 are formed on the bottom face of a ferroelectricsubstrate 201 as shown in FIG. 16( a). Subsequently, the substrate 201is put between electrodes 6 and 7 with sealing members 8 and 9 whileconductive liquid is filled between substrate 201 and each electrode 6and 7, and then, a voltage 4 is applied into electrodes 6 and 7 as shownin FIG. 2. Thus, ferroelectric spontaneous polarization reversal regions221 are formed in each region including the deepest part of each concaveportion as shown in FIG. 16( b).

The bottom face of the substrate in FIG. 16( b) where ferroelectricspontaneous polarization reversal regions are formed is cut and polishedto remove the concave portions. FIG. 16( c) shows the bottom face of thesubstrate 222 with said concave portions being removed.

As a method for removing the concave portions, filling said concaveportion with a material that has almost equal electrical resistivity ofthe substrate 201 could be a substitute method of the cut and polishing.

Next, the concave portions 223 are formed again on the bottom face ofthe substrate as shown in FIG. 16( d), and then, an electric field isapplied to form new ferroelectric spontaneous polarization reversalregions 224 as shown in FIG. 16( e).

Finally, the bottom face of the substrate is cut and polished to removethe concave portions 223. FIG. 16( f) shows the bottom face of thesubstrate 225 with said concave portions being removed.

With the above production processes, first ferroelectric spontaneouspolarization reversal regions 221 and second ferroelectric spontaneouspolarization reversal regions 224 are formed together on the substrate201. Thus, it is possible to form fine and close ferroelectricspontaneous polarization reversal regions.

In addition, it is also possible to form second concave portions orafter on the top face of substrate 201.

Next, another method for forming a fine and close a ferroelectricspontaneous polarization reversal region is explained.

Concave portions 230 and 231 are formed on the top face and bottom faceof the ferroelectric substrate 201 as shown in FIG. 17( a).

The substrate 201, where said concave portions are formed, is putbetween electrodes 6 and 7 with sealing members 8 and 9 while conductiveliquid is filled between substrate 201 and each electrode 6 and 7, andthen, a voltage 4 is applied into electrodes 6 and 7 as shown in FIG. 2.Thus, it becomes possible to form several ferroelectric spontaneouspolarization reversal regions 232 and 233 in the region including thedeepest part of each concave portion as shown in FIG. 17( b).

After that, by cutting and polishing the top face and bottom face of thesubstrate 201, the substrate where several ferroelectric spontaneouspolarization reversal regions are located closely as shown in FIG. 17(c) can be obtained. The concave portions used in forming a ferroelectricspontaneous polarization reversal are removed from the top face 234 andbottom face 235 of the substrate.

The shape of the concave portions according to the present invention isnot limited to the triangle shape at cross section as shown in FIGS. 15to 17, but can use various shapes at cross section such as an ellipse aslong as the width of said concave portions gets narrower toward theinside of the substrate.

The present invention is not limited to the above description, butrather, it is obviously possible to apply a well known technology in theart if necessary.

As described above, in accordance with the present invention, it ispossible to form a ferroelectric spontaneous polarization reversalcondition homogeneously within the ferroelectric spontaneouspolarization reversal region even across a large region of 50 μm andover in width of a ferroelectric spontaneous polarization reversal beingformed. Further, the present invention can provide a method for forminga ferroelectric spontaneous polarization reversal capable of loweringintensity of the applied voltage in a ferroelectric spontaneouspolarization reversal.

In addition, in accordance with the present invention, it becomespossible to provide a method for forming a ferroelectric spontaneouspolarization reversal where the ferroelectric substrate hasconvexo-concave structure such as ridge structure and the like on itssurface and a polarity of the region of the substrate including oneportion of said convex part is reversed with accuracy.

Moreover, in accordance with the present invention, it becomes possibleto provide a method for forming a ferroelectric spontaneous polarizationreversal capable of forming the fine and close ferroelectric spontaneouspolarization reversal region on the ferroelectric substrate, andfurther, capable of preventing a production process from beingcomplicated.

Also, since the method for forming a ferroelectric spontaneouspolarization reversal according to the present invention forms aferroelectric spontaneous polarization reversal using the concaveportion, it can be used for all types of substrate, and an insulatingmask or the like is not necessary. Thus, it has a great deal ofpotential in industry.

1. A method for forming a ferroelectric spontaneous polarizationreversal in a desired region of a ferroelectric substrate, comprisingthe steps of: forming, for said desired region of a surface of theferroelectric substrate, an electrode pattern or a mask pattern composedof aggregates of micropatterns, and then applying a given voltage intosaid desired region.
 2. The method for forming a ferroelectricspontaneous polarization reversal according to claim 1, wherein aconfiguration of the micropatterns is one of stripe-shaped pattern,ellipse-shaped pattern, hexagon-shaped pattern, network pattern, anddouble cross shaped pattern.
 3. The method for forming a ferroelectricspontaneous polarization reversal according to claim 1, comprising thesteps of: generating many nucleuses by using the electrode pattern orthe mask pattern composed of aggregates of micropatterns, forminganother electrode pattern or another mask pattern corresponding to saiddesired region, and then applying a given voltage into said desiredregion to generate a ferroelectric spontaneous polarization reversalaround the nucleuses.